Turbulence-chemistry interactions in the intermediate regime of premixed combustion

Abstract

A numerical model for the partially stirred reactor (PaSR) is developed, and the effects of turbulence on NO, CO, and other quantities are computed. Turbulent mixing is accounted for by the “Interaction-by-Exchange-with-the-Mean” submodel. The PaSR is described by a system of (2 N s+1) × N p first-order coupled o.d.e.'s in time, where N s ≡ number of species, and N p ≡ number of particles. Combustion of a 50% CO 50% H 2 (by vol.) fuel premixed with air is considered, represented by 18 species and 43 reactions. In the limit of mixing frequency ω becoming small, the solutions tend to those of the plug flow as expected. NO and CO increase with mixing frequency. In the range of time scales relevant to turbulent combustion, say, 10 −4 s < 1 ω < 10 −2 s, NO increases by a factor of about 2 as the mixing time becomes small enough to affect the concentration of oxyhydrogen radicals while CO increases by over an order of magnitude. These variations agree qualitatively with experimental data from turbulent combustors. In-combustor stirring clearly plays a large role even in premixed combustion. The algorithm converges to the perfectly stirred reactor solution at large mixing frequencies. The partial equilibrium model is found to be reasonable for CO H 2 fuels in the present range of conditions, and effects a computational speedup by a factor on the order of 100. Besides providing a useful combustion model, the PaSR provides a test-bed for mixing models, for simplified chemical schemes, and for algorithms intended for particle-tracking pdf transport models.